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I asked this question on the biology site. Some suggested I ask this question here.

https://biology.stackexchange.com/questions/95588/why-does-this-oxygen-duration-chart-have-increased-times-above-30-000-ft-msl

This oxygen duration chart from an airplane shows the amount of time a pilot can be on oxygen at a specified altitude (in 1000s of feet). For example, the chart shows two pilots able to cruise at a cabin altitude of 35,000 FT MSL for 182 minutes using a diluter demand system or 192 minutes using 100% oxygen.

I have one question.

  1. Why does the oxygen duration chart show increasing times above 30,000 FT MSL?

Oxygen Duration Chart

wbeard52
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  • Because the system delivers fewer standard liters per minute since the pressure is lower. Or, put differently, this is the opposite problem from scuba divers having less time at deeper depths. – Jon Custer Sep 08 '20 at 15:58
  • The bottle is regulated to output 75psi for all the crew and passengers. I did think of that but why starting at 30,000 FT? The human body needs the same amount of oxygen at any altitude, right? – wbeard52 Sep 08 '20 at 16:00
  • The masks are not delivering 75psi, so there is a pressure regulator downstream of it to keep the pressure at the mask slightly above ambient. – Jon Custer Sep 08 '20 at 16:02
  • Yes, I would agree with that. This specific mask will automatically provide positive pressure breathing above 37,000 FT MSL. I would imagine the time would decrease once that occurs. I do appreciate your help. – wbeard52 Sep 08 '20 at 16:06
  • But that 'positive pressure' decreases with increasing altitude. Just like the scuba regulator delivers more standard liters per breath at increasing depth (and ambient pressure) – Jon Custer Sep 08 '20 at 16:15
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    Seems like a question likely to gain more traction on [aviation.se]. – StephenG - Help Ukraine Sep 08 '20 at 16:19

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